1. Technical Field
This invention relates to a method and apparatus for measuring phase differences between intensity-modulated optical signals, especially, but not exclusively, for the determination of chromatic dispersion, polarization mode dispersion, changes in length/distance, and so on.
2. Background Art
It is often necessary or desirable to measure the relative group delay of intensity-modulated optical signals in elements of optical systems, especially, but not exclusively, optical fibers. Such measurements are necessary, for example, for evaluating such things as chromatic dispersion (CD), polarisation mode dispersion (PMD) or strain (fiber elongation). Of the various techniques for measuring relative group delay of optical signals, whether for distance measurements or for dispersion measurements, the most common involve the measurement of either time intervals between pulsed signals or phase differences between sine modulated signals, depending on how one modulates the intensity of the light. If one pulses the light, the time interval of interest is given by the measured time difference between a reference pulse (from a pulse generator or from a reference light pulse) and the optical signal pulse. There is uncertainty in quantifying the arrival times of the pulses because the pulses have a finite temporal extent and spreading of the pulses makes it difficult to detect them accurately. This, and jitter, render this technique usually less accurate than the direct measurement of phase shift.
Phase shift measurement techniques involve modulating the intensity of light from a light source (or from many light sources) using a sine modulated signal at a given sufficiently high frequency, typically at least 10 MHz for CD measurements. The, or each, resulting intensity-modulated optical signal usually has one major Fourier component. It travels through an optical path, is detected by an optical receiver and is transformed into an electrical signal. While travelling through the optical path, the optical signal is delayed and so, on arrival at the detector, has a phase difference with respect to a reference. During processing following detection, the resulting electrical signal also will experience additional delay relative to the reference.
The reference may be derived from the electrical signal used for the modulation of the light source(s)xe2x80x94usually from an electronic oscillator, or be derived from an electrical signal generated by a second optical signal (the modulation coming from the same reference oscillator) which has travelled a different optical path and has been detected by a second optical receiver. Time differences are obtained by determining phase differences between the two electrical signals, using electronic phase detectors.
In such known measurement methods, the time intervals or phase differences are measured in the electrical domain by an optical receiver that detects the modulated light and converts the optical signal into an electrical signal to be measured. The reference (from the reference oscillator or from the second optical receiver) and the signal to be measured do not follow the same electrical path (different path lengths, different gains, different filters . . . ), and the induced delays in the electrical domain are difficult to control or calibrate and are not related directly to the optical delays. Phase in electronic systems is especially difficult to stabilize, control or calibrate at high frequencies. Consequently, electronic phase errors add uncertainty to modulated optical signal phase shift measurements.
The present invention seeks to avoid these disadvantages and, to this end, in embodiments of the present invention two different optical signals intensity-modulated at the same frequency are permutated by a receiver to produce several different combination signals, converted to corresponding electrical signals at the frequency of modulation, and phase difference between the modulation of the two optical signals computed on the basis of trigonometrical relationships between the respective amplitudes of the combinations.
Because only the relative amplitudes of the electrical signals need be determined, the errors inherent in measuring pulse arrival time or electrical phase measurement can be avoided.
According to a first aspect of the present invention, a method of measuring phase differences between at least two optical signals both intensity-modulated at the same high frequency comprises the steps of sequentially selecting the two optical signals individually and in combination to produce a plurality of selected optical signals; deriving from the selected optical signals a corresponding electrical signal at the modulation frequency and having a series of different amplitudes corresponding to the different optical signal selections; and determining the different amplitudes; and
using trigonometrical relationships between amplitude and phase difference, computing from the determined amplitudes the phase difference between the modulations of the first and second optical signals.
For measurement of chromatic dispersion, the method may comprise the step of varying the wavelength of one or both of the two optical signals, measuring the phase difference at each of a plurality of selected wavelengths, and deriving chromatic dispersion from the resulting plurality of phase difference and wavelength measurements.
For measurement of group delay in dependence upon state of polarization, the method may comprise the step of varying the state of polarization of one of the two optical signals and measuring the phase difference for each of a plurality of different states of polarization.
For measurement of elongation, the method may comprise the steps of varying the effective optical length of an element in the propagation path taken by one of the two optical signals and measuring the phase difference for each of a plurality of different lengths.
The two optical signals may be generated by splitting intensity-modulated light from a single source and directed along different propagation paths. Alternatively, the optical signals may come from two or more different light sources. The light source may provide the intensity-modulation in response to an electrical modulation signal originating from a reference oscillator. Where a plurality of light sources are used, they may be controlled by a single reference oscillator providing several electrical signals phase-locked together. Yet another option is for the optical signals produced by the one or more light sources to be passed through an external optical intensity modulator.
Part of one optical signal may travel a third path, producing a third intensity-modulated optical signal having modulation at the same high frequency, and a known relative propagation delay with respect to one of the other optical signals, preferably about 90 degrees at the modulation frequency. The afore-mentioned plurality of selected optical signals then may comprise also the third optical signal selected individually and/or in combination with each or both of the first and second optical signals.
According to a second aspect of the invention, apparatus for measuring phase difference between intensity-modulated optical signals comprises:
means for providing a first optical signal and a second optical signal both having the same high frequency modulation; a selection unit for selecting sequentially the first and second optical signals individually and in combination; means for deriving from the selected optical signals a corresponding electrical signal having a series of different amplitudes corresponding to the different selections; means for determining the amplitudes; and means for computing from the amplitudes, using trigonometrical relationships between amplitude and phase, a phase difference between the first and second optical signals.
Embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings.